[css-anchor-position-1] Missing description of the `position-area` values `span-left` and `span-right`

[jfkthame]

This is presumably a simple editorial fix:

See https://drafts.csswg.org/css-anchor-position-1/#position-area-syntax

The syntax for position-area values includes span-left and span-right keywords, but these are missing from the following description of what the values mean. I would expect them to be there alongside span-top and span-bottom.

[fantasai]

Fixed, thanks!